The invention disclosed herein broadly relates to asset management systems, and more particularly to a system for tracking the real-time location and status of vehicles of a fleet, and for communicating between the vehicles and a dispatcher or expediter in the fleet offices.
Operators of fleet vehicle businesses need to know where each vehicle in the fleet is located and what it is doing in order to make decisions on how to use the vehicles most efficiently. In recent years, vehicle locating systems have been developed using Global Positioning System (GPS) satellite information, and, for greater accuracy, differential GPS (DGPS) systems. These systems are highly accurate where line of sight (LOS) conditions exist, that is, where the vehicle (or more accurately, the vehicle's GPS receiver) has a clear LOS to the appropriate number of GPS satellites. But such conditions are typically unavailable or are at least less frequently available for a vehicle operating on city streets, particularly in areas where multi-story buildings are present, owing to the shielding that such buildings effect. In those circumstances an alternative navigation system such as dead reckoning (DR) navigation may be used to provide vehicle position and velocity data in urban canyons (i.e., streets bordered by tall buildings) where GPS measurements are only intermittently available. Or a map matching technique or navigation grid may be used as another or additional alternative.
Currently, wireless voice communication between dispatchers and drivers is the primary means of addressing the need of the fleet owner or operator to know what each vehicle is doing, i.e., its operations taking place at any given time and where the vehicle is located when a particular operation is occurring. In industries where vehicles perform a repetitive sequence of events with each load, such as for ready mix concrete operations, “status boxes” have recently come into use. The status boxes require the driver to press a button at each stage of operation such as “load,” leave plant,” “arrive job,” “begin pour,” and so forth.
The primary problem with either wireless voice communication or status box systems is that data are manually provided to the dispatcher from the driver of the vehicle. This leads to untimely (late) and, perhaps worse, inaccurate data more than ninety percent of the time, according to analyses performed by the fleet owners/operators. The availability of timely, accurate data is essential if the fleet operator is to operate its business efficiently and economically.
Time Division Multiple Access (TDMA) wireless networks, which are in use for many applications including digital cellular telephones and wireless local area networks, may be used for the communication between dispatchers and drivers. A TDMA network allows multiple users of a single channel or frequency by assigning specific time slots to each user to use exclusively for transmission. For optimal performance of TDMA networks, precise time synchronization between members of the network is required. Efficient use of bandwidth in the network requires that the gap times between transmissions of each user, which is wasted time, be minimized. An important component to the gap time is uncertainty of time in all the participants in the network. Synchronization of wireless networks is often very coarse, requiring large gaps between transmissions, if specialized hardware is not used. Moreover, synchronization of network elements to a precise reference like GPS based timing information involves having a GPS receiver located on each network element, both mobile and fixed, increasing installation costs and complexity for both fixed network infrastructure and mobile network devices, especially if navigation data provided by GPS is not required.
Precise time synchronization between all of the wireless devices in the network can be performed in a number of ways. Typically, a precise, stable time reference, such as one based on the Global Positioning System (GPS) or other time distribution services, is located within each wireless device or within just the fixed infrastructure of the network, with synchronization information being transmitted to the mobile units. In these cases, device or infrastructure costs are increased because timing equipment has to be distributed among several locations or devices and installed where space and access for maintenance are limited.
Transmitting as much information as possible in a given amount of bandwidth is an important design goal in any communications network. This is especially true in wireless networks in which available bandwidth is very limited and customer requirements for data throughput are immense. Operation on most wireless bands is subject to occupied bandwidth constraints, requiring the data signal to be contained in a vary narrow region of the electromagnetic spectrum. In TDMA networks, a challenge is to minimize the gap times between transmissions and the overhead associated with each data packet in order to send as much information bearing data over the network as continuously as possible. The present invention addresses these two requirements with digital filtering to control occupied bandwidth and data recovery by the receiving system that requires no synchronization patterns to be transmitted.